1. Field of the Invention
The present invention relates to a slip control system for a vehicle and, more particularly, to a slip control system for the vehicle, adapted to prevent a slip of the driven wheels on a road surface from becoming excessive by controlling torque to be applied to the driven wheels.
2. Description of Related Art
Prevention of a slip of the driven wheels from becoming excessive is effective in order to efficiently provide a propulsive force of the vehicle as well as to provide safety in terms of prevention of a spin of the vehicle body and so on. In order to prevent an excessive amount of a slip of the driven wheels, torque to be applied to the driven wheels as a cause of slipping may be reduced.
Slip control of this type is disclosed, for example, in U.S. Pat. Nos. 4,484,280 and 4,583,611 (corresponding to Japanese Patent Unexamined Application (kokai) Nos. 16,948/1983 and 56,662/1985, respectively). They disclose technique of reducing the torque applied to the driven wheels by braking the driven wheels by brakes as well as reducing the torque to be generated by the engine. More specifically, U.S. Pat. No. 4,484,280 (corresponding to Japanese Patent Unexamined Application (kokai) No. 16,948/1983) discloses the technique of reducing the torque to be applied to the driven wheels only by braking the driven wheels when a slip of the driven wheels is small and by reducing the torque generated by the engine, on top of braking the driven wheels, when a slip of the driven wheels becomes larger. U.S. Pat. No. 4,583,611 (corresponding to Japanese Patent Unexamined Publication (kokai) No. 56,662/1985) discloses the technique in which, when one of the left-hand and right-hand driven wheels is larger in slippage than the other, only the driven wheel having a larger slip value is braked and, when both of the driven wheels are larger in slippage, the two driven wheels are braked while the torque to be generated by the engine is reduced. In summary, the technique as disclosed in the two prior patent publications as described hereinabove uses the braking of the driven wheel or wheels by the brake or brakes as a main means and the reduction in the torque generated by the engine as an auxiliary means.
Japanese Patent Unexamined Publication (kokai) No. 31,869/1988 proposes that, given an implementation of the feedback control in order to make the slip value of the driven wheels reach a predetermined target value, an excessive slip be reduced without delay by temporarily reducing torque applied to the driven wheels by a predetermined decrement when the slip value of the driven wheels becomes larger than a given judgment value larger than a predetermined target value.
Japanese Utility Model Unexamined Publication (kokai) No. 146,755/1983 proposes that, in order to ensure adjustment of the braking force by means of manual operation for a slip control system, the slip control with the aid of braking is inhibited when a foot brake is operated. The slip control system as disclosed in this publication performs the slip control by the brake control only. It should be noted, however, that the slip control system indicated in the Japanese Utility Model Unexamined Publication No. 146,755/1983 is designed so as to equally inhibit the slip control when the foot brake was operated so that the driven wheels may cause slipping in the transition period of time for operating the brake by manual operation, whereby the vehicle is found likely to become unstable. More specifically, suppose that the brake is operated by manual operation in such a state that the slip control is performed using the brake, the slip control is inhibited by operation of the foot brake so that the braking pressure may be caused to be reduced as a result of delay of a rise in the braking pressure with the aid of operation of the foot brake, thereby causing the driven wheel to slip again from time to time.
On top of that, when the slip control is designed to be performed using the brake, a cut valve is usually interposed in a braking liquid pressure piping system so as not to cause the braking liquid pressure to kick back automatically toward the side of the braking pedal. In this instance, if there would be a delay of the operation of the foot brake in timing from suspension of the slip control using the brake, i.e., from operation of opening the cut valve, the braking force for the driven wheel may temporarily be reduced, thereby causing the driven wheel to slip.